1. Field of the Invention
The present invention relates generally to servo control systems used for positioning read/write transducers in data storage devices, and more particularly, to providing precise characterizations of non-linearities in a position error signal during manufacturing test calibration, so that the servo control system can be programmed with the necessary algorithms and parameters to perform accurate, real-time linearization calculations of the position error signal in order to compensate for the non-linearities in the position error signal.
2. Description of Related Art
It is well known in the art to store data on magnetic or optical disk drives. Data is stored on a disk drive on one or more tracks of predetermined format disposed on a disk-shaped recording media. The data is written to and read from the tracks using one or more transducers, which typically comprise read/write heads. Reading data from a desired one of the tracks on the disk surfaces requires knowledge of the read/write head position relative to the track as the disk rotates and the head is moved across the disk, and requires precise centering of the head over the disk track. Conventionally, the read/write head is mounted on a head positioning assembly that is moved by a servo loop.
The servo loop controls movement of the head positioning assembly across the disk surface to move the read/write head from track to track (track seeking) and, once over a selected track, to maintain the read/write head in a path over the centerline of the track (track following). Centering the read/write head over a track permits accurate reading and recording of data in the track.
In most devices, the servo loop is a closed loop system that utilizes sampled position information, also known as servo information, obtained from the disk surface to provide feedback for the track seeking and track following functions. Some devices store this position information between the data regions of the disk surface (known as an embedded servo system).
The sampled position information usually includes: a synchronization field, such as for automatic gain control (AGC) or similar signal detecting purposes; a track identification (TID) field typically comprising a digitally encoded Gray code; and a position error signal (PES) field generally containing one or more burst patterns. The PES, which is proportional to the relative difference of the positions of the center of the read/write head and the nearest track center, is a corrective signal providing an indication of which direction the head should be moved to during either track seeking or track following functions.
Linearization has been used to compensate for non-linearity in the PES. Consider, for example, U.S. Pat. No. 5,825,579, issued Oct. 20, 1998, to Cheung, et al., and entitled xe2x80x9cDisk drive servo sensing gain normalization and linearization,xe2x80x9d which patent is incorporated by reference herein. This patent describes a method for generating a continuous and linear PES from stitched components.
Generally, U.S. Pat. No. 5,825,579 covers the same field as the present invention, and provides a good introduction into the background, the problem being solved, and general approaches to solving the problem. Specifically, it describes a normalization stage in the servo loop that is used to correct discontinuities at the PES stitching points (i.e., quarter-track positions), to normalize the PES at the stitching points and track-center position, and then to obtain a smoothing function for the PES between the stitching points and track-center position. However, PES linearization in U.S. Pat. No. 5,825,579 is limited in that it is achieved only if the following assumptions are true: there is a triangular shape to the PES non-linearity and the shape is symmetrical about track-center, quarter-track and xe2x85x9th track positions.
There is a need in the art for improved methods of linearizing the PES due to the increasing significance of the problem with the increasing data storage radial density by increasing track-per-inch (TPI). A radial off-track position of the read/write head has been traditionally assumed to have a linear relationship to the PES. However, for magneto-resistive (MR) and giant magneto-resistive (GMR) heads, the head width is typically designed to be narrower than the track width, thereby causing the assumption to be inaccurate, with the result being errors in the position feedback, which contributes to track misregistration (TMR) and lower servo loop stability margins, and contributes to off-track read and off-track write events. The effects of PES non-linearity, if not properly compensated, are more significant with radial densities exceeding 50,000 TPI. With increasing TPI, the associated improvements in PES linearization methods must tackle challenges of measurement and calculation accuracy and reliability, and overcome hardware imperfections such as head magnetic asymmetries.
In the present invention, PES linearization compensates for the non-linearity in the translation of the PES to an off-track position of the head during each servo sector sampling that measures the PES. A real-time linearization calculation performed by the servo loop requires prior characterization of the non-linearity, which is performed during the calibration phase of each drive in the manufacturing process.
To minimize the limitations in the prior art described above, and to minimize other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method for precisely characterizing non-linearity in a position error signal (PES) for a data storage device during a calibration phase in the manufacturing process. From this characterization of the PES non-linearity, the data storage device can be programmed with the necessary data to perform accurate, real-time linearization calculations of the PES in order to compensate for non-linearity in the PES during operation. The sequence of calibration steps includes a PES gain calibration, a first servo loop gain calibration, a PES linearization calibration, and an optional second servo loop gain calibration.